Change cancel lesson button text

aleksis/apps/chronos/frontend/messages/en.json

@@ -55,8 +55,8 @@
           "groups": "Filter by groups"
         },
         "cancel": {
-          "cancel": "Mark lesson as cancelled",
-          "decancel": "Mark lesson as not cancelled"
+          "cancel": "Mark as cancelled",
+          "decancel": "Mark as not cancelled"
         }
       }
     }